Regioselective Cleavage of 3,4-Epoxy Alcohols with Substituted Alkynylaluminum Reagents: Application to the Stereoselective Synthesis of Polypropionates.

The reaction of 3-O-substituted propynyl aluminum reagents with a TIPS-protected 2,3-epoxy alcohol and several diastereomeric 2-methyl-3,4-epoxy alcohols offers a convenient synthetic approach for the subsequent preparation and epoxidation of allylic alcohols. The yields are low to moderate and the regioselectivity (internal vs external attack) similar to that of the standard diethylpropynyl aluminum reagent. The TMS-acetylide alane reagent gives improved yields and good regioselectivity, and constitutes the reagent of choice for the cleavage of the hindered epoxides. This attractive methodology is applied to the elaboration of two advanced polypropionate precursor fragments.